1. Field of the Invention
This invention relates to a simple process for pretreating triglycerides, especially natural fats and oils, for reducing their troublesome content of free fatty acids. The process of the present invention is particularly intended as a preparatory treatment for subsequent conversion of the triglycerides by transesterification with lower monohydric alcohols, particularly methanol, into glycerine and the corresponding fatty acid alkyl esters.
2. Description of Related Art
Fatty acid methyl esters have acquired considerably commercial significance as starting materials for the production of fatty alcohols and other oleochemical products, such as ester sulfonates, fatty acid alkanolamides and soaps. Industrially, fatty acid methyl esters are produced mainly by catalytic transesterification (alcoholysis) of fatty acid triglyceride mixtures of the type present in fats and oils of vegetable and animal origin.
Natural fats and oils almost always contain considerably quantitites of free fatty acids. their content of free fatty acids varies over a wide range, dependign on the origin of the material and its previous history, and almost always exceeds about 3% by weight.
Various processes are available for the transesterification of naturally occurring fatty acid triglycerides with alcohols. The choice of transesterification reaction conditions depends to a large extent upon the quantity of free fatty acids presesnt in the triglyceride mixture.
Atmosphere transesterification of fats and oils to form the corresponding fatty acid ester mixture may be effected with a 0.5 to 1.0-molar excess of alcohol in the presence of an alkali catalyst and at a temperature of between 25.degree. to 100.degree. C. Such a process is described in U.S. Pat. No. 2,360,844 as the first stage of a soap manufacturing process. This alkali-catalyzed, atmospheric transesterification process may be carried out without any problems as long as the starting materials used are fats and oils which are substantially free from water and which have a free fatty acid content ofless than 0.5% by weight (corresponding to an acid number of about 1).
Fats and oils having a relatively high content of free fatty acids may be transesterified in a high pressure transesterification process with a 7- to 8- molar excess of methanl in the presence of alkali or zinc catalysts to form the corresponding fatty acid methyl esters. This process is carried out at a temperature of 240.degree. C. and at a pressure of about 100 bar. (Ullmann, Enzyklopadie der technischen Chemie, 4th Edition, Vol 11 (1976), page 432).
Compared with high-pressure transesterification, atmospheric transesterification uses considerably less methanol and, by virtue of the lower reaction temperatures, less energy. In addition, atmospheric transesterification does not require expensive pressure reactors.
Commercial grade fats and oils, however, almost always contain relatively large quantities of water and free fatty acids. As a result, atmospheric transesterification of these commercial triglyceride mixtures requires preliminary drying and a reduction in the acid number, for example by conversion of the free fatty acids into the corresponding alkyl or glycerol esters in a preliminary or pre-esterification reaction. Pre-esterification of the acid-containing fats and oils may be carried out in the presence of alkaline cataylsts at a temperature of 240.degree. C. and at a pressure of 20 bar. (Ullmann, Enzyklopadie der technischen Chemie, 4th Edition, Vol. 11 (1976), page 432). This method of pre-esterification with methanol also requires the use of expensive pressure reactors.
It also is known that the free fatty acid content of oils can be esterified with lower monohydric alcohols, especially methanol, in the presence of an acid catalyst such as p-toluene sulfonic acid in a homogeneous phase reaction. Unfortunately, in order to prepare the pre-esterified product for subsequent alkali catalyzed transesterification, this process requires a relatively complicated separation of the catalyst and also the removal of water by washing the esterified oil with methanol. It is absolutely essential that the catalyst--normally p-toluene sulfonic acid--be carefully removed because any catalyst residue in the methyl ester during its subsequent reaction to form a fatty acid alcohol also can inhibit the hydrogenation catalysts. In order to separately remove the methanol wash containing the recovered water and acid catalyst, a liquid extractant which is substantially immiscible with the oil phase must be added to the oil for pre-esterification. Glycerine preferably is used as the immiscible extractant. Such a process is described in German application Pat. No. 33 19 590. Unfortunately, a portion of the esterified free fatty acids are lost in this known process.
An object of the present invention is to provide an improved process having all the advantages of the acid-catalyzed pre-esterification process, including in particular compratively mild reaction temperatures and pressures, but which avoids the difficulties involved in the reliable removal of undesirable acidic catalyst residue from the pre-esterified material.